It is much easier for a small animal to run uphill than for a large animal, because

It is much easier for a small animal to run uphill than for a large animal because the efficiency of muscles in large animals is less than in small animals. The metabolic rate is inversely proportional to the body size of the animal. Smaller animals have a higher metabolic rate per unit of body mass. So small animals can run faster and can climb uphill faster than large animals.